Electrically conductive shoe insoles are used to keep static electrical charges from building up on a worker's body. Friction can develop between a person's hands and feet when in contact with other surfaces, and this can cause an accumulation of electrostatic charges as the person moves about. These electrostatic charges can pose a serious threat of injury if the air contains combustible gases or flammable liquids, as in a munitions factory. The build up of electrostatic charges on the body can cause a spark when a worker touches a grounded metal object. Depending on the conditions, the spark may cause the airborne combustibles to explode.
To prevent explosions, workers in combustible-laden work areas normally wear electrically conductive shoes. These shoes keep electrostatic charges from accumulating by providing a conductive path of relatively low electrical resistance from a worker's foot to the floor. Electrically conductive insoles must be used in the electrically conductive shoes to maintain the conductive path. This allows the electrical charges to be transferred from the user's foot to the conductive shoe. Patents which describe various types of such shoe insoles include: U.S. Pat. Nos. 4,150,418 issued to Berbeco; 4,642,912 issued to Wildman et al. and 4,926,570 issued to Fohst; as well as Japanese patents 88-039658/06 issued to Kokoku and 52-32744 issued to Sato.
It has been discovered that conventional electrically conductive insoles progressively lose their ability to conduct electrical charges through a process known as degradation. Such shoe insoles typically consist of a base layer rendered electrically conductive and an upper layer of conductive fabric laminated to the base layer with an electrically conductive adhesive. Degradation is apparently caused by perspiration from the user's foot seeping into the fabric. The perspiration may corrode metallic conductive elements or cause the electrically conductive adhesive to crepe and exude through the fabric. Either effect increases the overall electrical resistance of the shoe insole and lowers its ability to conduct electrical charges.
Degradation of an electrically conductive shoe insole is often subtle but can become lethal. It usually starts slowly, then accelerates rapidly as the electrically conductive adhesive begins to exude through the fabric. If undetected, this degradation may cause a sudden, spark-induced explosion.
Consequently, there is a need for an improved shoe insole which can maintain its electrically conductivity over time and which can resists degradation from human perspiration.